1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and in particular, to a method for efficiently fabricating solder bumps on, for example, electrode pads of a semiconductor chip. In particular, the present invention relates to a method for fabricating solder bumps by forming solder balls from a solder paste.
2. Description of the Related Art
Recent data processing apparatuses include many LSIs and VLSIs for dealing with a large volume of data at a very high speed. LSIs and VLSIs comprise semiconductor chips which include integrated electronic circuits and parts. In order to mount a semiconductor chip to, for example, a ceramic substrate, solder bumps are arranged onto one of the semiconductor chip and the ceramic substrate so that the semiconductor chip can be mechanically and electrically connected to the ceramic substrate by melting the solder bumps.
Conventionally, a solder paste is printed onto the ceramic substrate, and leads extending from the semiconductor chip are placed on the solder paste on the ceramic substrate. The ceramic substrate is then heated to cause the solder to melt so that the semiconductor chip can be mechanically and electrically connected to the ceramic substrate (For example, Japanese Examined Patent Publication (Kokoku) No. 53-3980). For printing the solder paste onto the ceramic substrate, a metal mask having predetermined openings is used, so that the solder paste is applied to the surface of the metal mask and a squeegee is moved along the surface of the metal mask to cause the solder paste pass through the openings in the metal mask.
Japanese Unexamined Patent Publication (Kokai) No. 6-124953 discloses a method for forming solder bumps with the use of a plate having a plurality of cavities. In this method, the cavities are filled with solder paste, and a semiconductor device having gold bumps is moved toward this plate so that the gold bumps are stuck in the solder paste in the cavities. The semiconductor device is then moved away from this plate, and the solder bumps in the form of solder paste are attached to the gold bumps of the semiconductor device.
However, as the electronic circuits on the semiconductor chip are highly integrated, the number of the leads acting as input and output terminals increases and the pitch between the leads is shortened. In this situation, the amount of the solder paste to be printed becomes smaller and the printing of the solder paste becomes difficult. That is, the solder paste comprises solder powder usually having the powder size of 30 to 50 .mu.m and a flux including a resin, an activator, and an organic solvent, and the viscosity of the solder paste at the room temperature is relatively high, for example, in the order of 20 to 30.times.10.sup.4 cp. Therefore, if the size of the openings of the metal mask is small, for example 100 .mu.m, the solder paste may clog the openings of the metal mask and may not pass smoothly through the openings of the metal mask, with the result that the solder paste is not printed onto the ceramic substrate.
In the recently introduced flip-chip structure, leads are not attached to the semiconductor chip, but solder bumps are arranged on the semiconductor chip so that the semiconductor chip can be directly mounted on the ceramic substrate. In this case too, as the size of the solder bumps has become smaller, the printing of the solder bumps has become difficult. Thus, methods for fabricating smaller solder bumps, which do not use a solder paste but use solder only, have been proposed. One is a vacuum vapor deposition method, and the other is a solder plating method.
A vacuum vapor deposition method is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 4-014834. The solder is heated and vaporized in a vacuum environment, and the vaporized solder is deposited on an object. In the method disclosed in the above publication, a transfer plate made from a material such as quartz or glass having a good heat resisting property and a low wettability to the solder, and a metal mask having openings in a predetermined pattern are used. The transfer plate is placed above the metal mask so that the solder vapor which passes through the openings of the metal mask is deposited onto the transfer plate. Therefore, solder bumps are formed onto the transfer plate in the same pattern as that of the openings of the metal mask.
The heated semiconductor chip is then superposed on the transfer plate so that the solder bumps are transferred from the transfer plate to the semiconductor chip. In this way, solder bumps are formed on the semiconductor chip in the predetermined pattern. By this method, it is possible to form solder bumps with a high accuracy, but there is a problem that a long time is necessary for vaporizing the solder and forming solder bumps having a desired thickness, for example, several tens of .mu.m.
In the solder plating method, the semiconductor chip is subjected to plating after the surface of the semiconductor chip is covered by a resist and openings are formed in the resist at positions where solder bumps are to be formed. In this method, however, the circuits in the LSI, for example, are affected by the plating solution and undergo a chemical change, resulting in a deterioration of the electrical characteristic or an undesirable composition change in the deposited solder.
In addition, the solder bumps are formed not only on the semiconductor chip but also on other members such as a ceramic substrate or a ceramic package. Also, solder bodies in the form of balls without solder paste are available for certain applications. It is possible to form solder bumps using these solder balls. However, there is no prior art by which the solder balls can be easily fabricated with a very small variation in size.